But the refused to die.
In the RetroArch Quick Menu (Hotkey + X), go to Options and turn on "Skip Warnings" . This removes the red "This game is not perfect" text that MAME 2003 shows for every single game. MAME 2003 Reference Set - MAME 0.078 ROMs- CHDs...
CHDs are finicky. To play a game like Killer Instinct , you need the kinst.zip (the brain) in your ROM folder and the kinst.chd (the body) inside a subfolder named exactly /kinst/ . But the refused to die